Add a background color to the 0x and # part of an hexadecimal color value. The background color is extracted from the hexadecimal value itself as a reminder for this color.
Colors in string should be parsed as well.

I looked at the plugin API to find a way to implement it as a plugin but it seems that there's no way to simply affect a custom background color to a region (If i'm right, a region is associated with a scope which provides the colors and scopes are defined as part of syntax definition)